Primavera P6- duration in the summary bar not match with start and finish dates

this depends on which version you are using if you are usin 6.2 then Summary duration is calculated as the time between the earliest Start date of the included activities to the latest Finish date of the included activities according to the Global default calendar.  When multiple activities are included in a band, the summary duration is rarely equal to the longest activity duration unless all the activities occur simultaneously and the activity calendar assignments are all set to the Global default calendar.

if you are using 7.0 and above then 

In a single project, if all activities under a particular grouping are using the same calendar, the summary band will calculate duration based on that calendar.

or 

In a single project, if activities under a particular grouping are using a different calendar, the summary band will calculate duration based on the Project Default Calendar, set under Enterprise, Projects, Defaults tab.

its probally using the default calendar